“CBA currently controls approximately 25 per cent of the mortgage market and we expect the acquisition of Aussie Home Loans, which the ACCC said has around 6 per cent market share of the broker market, to cement its market-leading position," Moody’s senior vice president Ilya Serov said in the report.

CBA has agreed to pay around $270 million to increase its existing one-third stake in Aussie to 80 per cent, with a right to acquire the remaining 20 per cent in 2016.

The ACCC gave the deal the green light, despite acknowledging that it would allow CBA to sell more “white label" loans – mortgages that are financed by CBA but sold under the Aussie Home Loans brand.

Moody’s said some borrowers would not be aware of the link between CBA and Aussie.

“Controlling Aussie Home Loans offers CBA a number of auxiliary benefits, particularly the ability to offer white-label Aussie Home Loans-branded loans that enables CBA to better differentiate its product offering in different market segments," Mr Serov said.

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“Industry research shows that customers typically have little appreciation for the ultimate ownership structure of lending and brokerage groups.

“We expect the Aussie Home Loans-branded product to attract those customers who are traditionally less inclined to choose a major bank lender. The deal also offers CBA the ability to differentiate pricing between branch and broker-originated customers, and, consequently, more finely manage its margins."